1. Field of the Invention
This invention relates to an optical pickup for recording and/or reproduction of an optical recording medium, a reproducing apparatus and a recording apparatus for an optical recording medium. More particularly, it relates to an apparatus capable of recording and/or reproducing plural sorts of optical discs different in track pitch by one and the same apparatus.
2. Description of the Related Art
Heretofore, in an apparatus for reproducing an optical disc, such as a compact disc, a three-beam method has been used as a system for generating tracking error signals. This system splits a light beam radiated by a semiconductor laser element by a diffraction grating into three beams, namely a main beam and two side beams. The main beam is radiated on a recording track of an optical disc, while both side beams are radiated on the positions of the disc offset by one-fourth of a track to both sides of the recording track. The light beams radiated on the optical disc are reflected by the recording surface of the recording medium so as to be received by a photodetector. This photodetector is made up of a first photodetector portion for receiving a main beam and second and third photodetector portions for receiving both side beams. The tracking error signal is detected by finding the difference between the signals received by the second and third photodetector portions.
Recently, an optical disc has been investigated in which data may be recorded to a high density for recording data of high precision, such as still pictures or moving pictures. With such an optical disc, it may be contemplated to set the track pitch to approximately 0.8 .mu.m instead of to 1.6 .mu.m conventionally used, or to form a recording layer of a narrower track pitch as multiple layers. The optical disc, recorded to a high density, is not limited to a replay-only type, but a rewritable optical disc, such as a phase change type disc, is also contemplated. Such a disc having a guide groove may also been envisaged as such rewritable optical disc.
However, it is difficult with an optical disc for high density recording to detect tracking error signals using the above-mentioned three-beam system. That is, since the track pitch is of a narrow width, registration of the side spots of the three spots radiated on the recording surface of the optical disc becomes difficult. Also, if the high-density recording layer is formed as multiple layers, a problem arises that an offset is produced in the tracking error signal due to leakage of the reflected light from the layer other than the layer being recorded or reproduced. In addition, if the rewritable optical disc is a phase change type disc, and recorded and unrecorded portions are produced on the disc, a noise is produced with the three-beam method due to differences in reflectance in the recorded and unrecorded portions, thus making it difficult to generate correct tracking error signals.